1. Field of the Invention
This invention relates to a lead frame, more particularly to a multi-layer lead frame having at least one metal plate or plane, hereinafter referred to as "metal plane" which is used for a semiconductor device. This invention also relates to a process for manufacturing such a multi-layer lead frame.
2. Related Art
A plastic package, whose speed and heat-radiation characteristics are as good as those of a ceramic package, has recently been developed. In addition, in place of a conventional single-layer lead frame, a multi-layer plastic lead frame for a semiconductor device has also been developed.
Such a multi-layer lead frame comprises at least one metal plane connected via an insulative layer to the bottom surfaces of inner leads. Using such a multi-layer lead frame, it will be possible to mount a relatively power-consumable semiconductor chip on the metal plane, due to a good heat radiation capacity of the metal plane. Also, the metal plane can be used as a ground layer or a power supply layer to improve the electrical characteristics of the semiconductor device.
FIG. 10 shows a conventionally known multi-layer lead frame which comprises three layers, i.e., a signal layer (i.e., lead frame 12) and two metal planes, i.e., a power supplying metal plane 14 and a ground metal plane 16, which are laminated via insulative tape 17a and 17b made of heat-resistant material made of, such as a polyimide.
Using such a multi-layer lead frame, it would be possible to prevent a so-called cross talk which might be generated between signal lines and also possible to reduce the capacitance between the lines or inductance.
As shown in FIG. 10, in order to manufacture the abovementioned multi-layer lead frame, at the first stage, a lead frame 12, a power supplying metal plane 14, a ground metal plane 16, and insulative tape pieces 17a and 17b each having respective surfaces providing with adhesive layers must individually be made. Then, these individual pieces must be positioned by using a special instrument (not shown), such as an image reader, and then heat-pressed to laminate and adhere them together.
However, in the above mentioned manufacturing process, it has been very troublesome and laborious work to position and laminate the five layers including the insulative tape pieces 17a and 17b by using the image reader. It also requires a lot of time and, therefore, an effective production rate would not be expected. In addition, after these pieces are gripped by the gripping means and laminated, when the pieces are released or disengaged from the gripping means, the laminated pieces might be moved slightly and, therefore, a precise laminated multi-layer lead frame might not be obtained.
The multi-layer lead frame should originally have a good quality for multi-pin and, therefore, inner leads thereof are usually very dense. Thus, even a slight abberation might cause a disconnection between the terminals of the power supply plane 14 and ground plane 16 and the corresponding inner leads of the lead frame 10.
In addition, an insulative sheet must be punched to form the insulative tape pieces 17a and 17b. Such a punching process requires a relatively laborious work.